One of the requirements of an efficient engine is the correct amount of heat shock, delivered at the right time. This requirement is the responsibility of the ignition system. Digital ignition system is same as an electronic ignition system in which spark timing is controlled by a computer that continuously adjust ignition timing to obtain optimum combustion.
The digital spark ignition is the best alternative for conventional ignition system. Computerized control gives accurate timing for all operating conditions. At the same time use of two spark plugs improves thermodynamic efficiency and power available.
It is also good solution to reduce pollution since it minimizes emission levels. Also it is flexible enough in mounting location. This is important because today’s smaller engine compartment.
Thus it is better in all areas like power, speed, efficiency and clean emission and hence it has brought a new evaluation in automobile industry.
It must generate an electrical spark they have enough heat to ignite the air fuel mixture in the combustion chamber.
It must maintain the spark long enough to allow for the combustion of all the air-fuel mixture in the cylinder.
It must deliver the spark to each cylinder so combustion can begin at the right time during the compression stroke to each cylinder.
When the combustion process is completed, a very high pressure is exerted against top of the piston. This pressure pushes the piston down on its power stroke and is the force that gives the engine power. For an engine to produce maximum power it can, the maximum pressure from combustion should be present when the piston is at 10 to 23 degrees after the top dead centre. Because combustion of air-fuel mixture within the cylinder takes short period of time, usually in milliseconds, the combustion process must begin before the piston is on its power stroke. Therefore, the delivery of the spark must be timed to arrive at some time before the piston reaches the top dead centre.
1.2 Conventional Single Spark Plug Ignited Four-Stroke Engine
The orthodox single cylinder, four-stroke, spark ignition engine is generally equipped with a single spark plug. The fresh charge (air – fuel mixture) that entered the cylinder during the suction stroke is compressed during the compression stroke resulting in the increase of pressure and temperature of the charge. The spark plug, usually situated at one end of the combustion chamber, ignites the air-fuel mixture and the ensuing flame spreads like a slowly inflating balloon. There is an inevitable delay for this inflating balloon to reach the furthest part of the combustion chamber. So, there are pockets of poor combustion within the chamber and, overall, the combustion is slow and inefficient. But there are some problems that single spark flame propagation cannot give optimal combustion hence DTSi system is invented and Patented by Bajaj Auto.
The Digital Twin Spark Ignition technology, patented by Bajaj Auto, takes care of this slower combustion problem in a simple way. The cylinder head is equipped with two spark plugs, instead of the usual one. By generating two sparks at either ends of the combustion chamber, (approximately 90° to the valve axis) the air-fuel mixture gets ignited in a way that creates two flame fronts and, therefore, a reduction in flame travel of the order of 40 per cent is achieved. A fast rate of combustion is achieved leading to faster rise in pressure. The obvious outcome of this is more torque, better fuel efficiency and lower emissions. An electronic device (microprocessor) controls the firing order of these twin spark plugs. The fresh charge that entered the cylinder during the suction stroke is compressed during the compression stroke. Then a spark will be ignited by one of the twin spark plugs and the flame front begins to expand like an inflating balloon. In the mean while another spark will be ignited by another spark plug as per controls of the microprocessor. The flame front also begins to expand like an inflating balloon. Therefore the areas that are not covered by the first flame front will be covered by second flame front resulting in the complete & rapid combustion of the fuel. This technology is incorporated in Bajaj bikes since 2003, in series of Pulsar and Discover.
Ignition timing refers to the precise time spark offers and is specified by referring to the position of the piston in relation with crank shaft rotation. Ignition timing reference marks can be located on engine parts and on a pulley or fly –wheel to indicate the position of the piston. Vehicle manufacturer specify initial or base ignition timing. When the marks are aligned at TDC or 0, the piston in cylinder is at TDC of its compression stroke. Additional numbers on a scale indicates the number of degrees of crank shaft rotation before TDC or after TDC. In majority of engines, the initial timing is specified at a point between TDC and 20 degrees BTDC.
If optimum engine performance is to be maintained, the ignition timing of the engine must change as the operating condition of the engine change. All the different operating conditions affect the speed of the engine and load on the engine. All ignition timing changes are made in reference is made to following primary factors:
At higher rpm, the crank shaft turns through more degrees in a given period of time. If combustion is to be completed by 10 degrees ATDC, ignition timing must occur sooner or advanced however, air–fuel mixture turbulence increases with rpm. This causes the mixture inside cylinder to turn faster. Increased turbulence requires that ignition must occur slightly later or be advanced. These two factors must be balanced for the best engine performance. Therefore, while the ignition timing must be advanced as engine speed increases, the amount of advance must be decreased some to compensate for the increased turbulence.
Figure 2: Effect of speed on ignition timing (at High Speed & at Low Speed)